Battery operated rechargeable appliances usually employ nickel cadmium rechargeable batteries as a power source. Nickel cadmium batteries can be recharged many times between use and are not damaged by high-rate deep discharge. In addition, these batteries are small and can be used to power relatively large loads.
The permissible charging rate in nickel cadmium batteries varies widely with temperature. In the vicinity of normal room temperature (20.degree.-35.degree. C.) a typical 1/3AA nickel cadmium battery can be charged at a continuous overcharge rate of about 30 milliamperes. At low temperature the allowable charge rate drops to 2 milliamperes and at high temperatures of around 65.degree. C., the charging rate drops to about 10 milliamperes. Normal household appliances encounter temperatures in the 20.degree.-35.degree. C. range and the higher charging rate of 30 milliamperes is appropriate.
The automotive temperature environment has prevented the use of rechargeable nickel cadmium batteries due to the wide variation in permissible charging rates required by the great temperature ranges normally encountered in vehicles, even though an appropriate source of power is readily available in the automotive storage battery and its generator or alternator supply.